1. Technical Field
This invention relates to the art of radio broadcasting and, more particularly, to an AM (amplitude modulation) transmitter with a tracking power supply for its amplifiers.
2. Description of the Prior Art
The U.S. Patents to H. I. Swanson U.S. Pat. No. 4,580,111 and 4,949,050 disclose an amplitude modulator for use in AM radio broadcasting and wherein the modulator serves to generate an amplitude modulated signal by selectively turning on and off a plurality of RF amplifiers in digital manner to produce amplitude modulation. Each of the RF amplifiers provides an output voltage across the primary winding of a transformer. The secondary windings of the transformers are connected together in series in a series combiner. Output current flows in the secondary windings to supply a load, such as an antenna, for broadcasting an RF signal.
In a system as noted above, the level of modulation attainable by the transmitter is limited by the number of amplifiers in the system. While the level of modulation can be increased by adding additional amplifiers to the system, this constitutes an added expense in the system design. The added expense is especially unfortunate, given the fact that the new amplifiers will be needed only for the occasional peak signal. Often, the additional amplifiers will contribute only a small amount to the average power output of the transmitter. This is an inefficient use of the design of the amplifier that unnecessarily increases the expense of the transmitter system.
In accordance with one aspect of the present invention, an AM transmitter is disclosed with tracking power supply compensation. A first power supply provides a constant DC operating voltage. A second power supply is responsive to an input signal, such that the second power supply provides a DC voltage when an amplitude associated with the input signal exceeds a predetermined level. N RF amplifiers amplify the input signal. Each of the plurality of amplifiers are connected to the first and second power supplies. Each of the amplifiers has an output circuit that, when the associated amplifier is on, supplies energy from the first and second power supplies to provide an output voltage to a load.
In accordance with another aspect of the present invention, an AM transmitter is disclosed with tracking power supply compensation. A first power supply provides a constant DC operating voltage. A second power supply is responsive to an input signal, such that the second power supply provides a DC voltage when an amplitude associated with the input signal exceeds a predetermined level. N RF amplifiers amplify the input signal. Each of the plurality of amplifiers are connected to the first and second power supplies. Each of the amplifiers has an output circuit that, when the associated amplifier is on, supplies energy from the first and second power supplies to provide an output voltage to a load. A controller selectively turns on one or more of the N amplifiers with the number that are turned on being a function of the input signal.